1. Field of the Invention
This invention relates to a universal joint for use in a power transmitting system for a vehicle or the like.
2. Description of the Prior Art
An example of this type of universal joint has been known which is provided with a joint body having an inner ring disposed at one end of a first shaft and an outer ring disposed at one end of a second shaft, and a flexible boot formed from a high elasticity material such as rubber or resin and fitted at its opposite ends to the outer ring and the first shaft. In this universal joint, grease for lubricating the joint body is enclosed inside the flexible boot, and a sealed space which is charged with air is formed in the flexible boot. The flexible boot retains the grease in a sealed manner and functions to prevent any dust or water from entering into the sealed space from the outside.
The rubber or resin which form the above flexible boot are materials having a gas-permeable property, and the rate of gas permeation through such material increases in response to any increase in the pressure differential between the inside and the outside surfaces of the material and to any rise in temperature. Therefore, in the above-described type of conventional universal joint having a sealed space charged with air, when the internal pressure of the sealed space is increased due to heat evolution from the joint caused when the joint moves or due to a rise in the ambient temperature, air in the sealed space escapes to the outside by permeating through the flexible boot. As the temperature of the joint decreases after movement of the joint ceases, a phenomenon occurs whereby outside air enters the sealed space by permeation through the flexible boot. As described above, the rate of gas permeation increases in response to any rise in temperature, and the gas permeation rate displayed when the joint temperature is low during the time when the universal joint is in a stationary state is very much smaller than that displayed when the joint temperature is high at the time when the universal joint is in a state of movement. Consequently it takes a much longer time for a certain volume of air to enter the sealed space compared with the time taken for the same quantity of air to escape from the sealed space. Accordingly, as the operations of moving and stopping the universal joint are repeated, the internal pressure of the sealed space decreases gradually, and the bellows portion of the flexible boot is correspondingly deformed inwardly in the diametral direction. If the operations of moving and stopping the universal joint are further repeated while the flexible boot is being deformed in this way, there is a risk of the inner end of the inwardly deformed portion being nipped in the joint body and thereby being damaged.
In the conventional universal joint whose sealed space is charged with air, there is also a problem of the grease enclosed in the sealed space deteriorating due to chemical reaction between the grease and oxygen in the air, along with a problem whereby the extent of the above-described deformation of the flexible boot is further increased as the negative pressure in the sealed space decreases due to the reduction in the volume of air brought about by the above reaction and by absorption of oxygen by the material of which the flexible boot is made.